1. Field of the Art
This invention relates to crosslinkable polyethylene resin compositions. More particularly, it relates to a water-crosslinkable polyethylene resin composition which comprises a copolymer of ethylene and an unsaturated silane compound having hydrolyzable silane groups as the crosslinkable group and a catalyst for crosslinking reaction.
2. Prior Art
A polyethylene resin such as low-density polyethylene and the like has been generally crosslinked to enhance its mechanical strength and heat resistance.
As one of such crosslinking measures, it has been known that the polyethylene to be used can be made crosslinkable by introduction of a crosslinkable group and, in this case, a hydrolyzable silane group is utilized as the crosslinkable group. For example, (i) as described in Japanese Patent Laid-open Publn. No. 9611/1980, a copolymer of ethylene and an unsaturated silane compound represented by the formula RSiR'.sub.n Y.sub.3-n (wherein: R is an ethylenically unsaturated hydrocarbyl or hydrocarbyloxy group; R' is an aliphatic saturated hydrocarbyl group; Y is a hydrolyzable organic group; and n stands for 0, 1 or 2), or (ii) as described in Japanese Patent Publication No. 1711/1973, an unsaturated silane compound-graftpolymerized polyethylene prepared by graftpolymerization of an unsaturated silane compound onto polyethylene in the presence of a free radical generator, is crosslinked by contacting it with water in the presence of a silanol condensation catalyst.
The above described methods are advantageous in comparison with other conventional crosslinking procedures such as a method using irradiation with radiation rays or a method utilizing decomposition of organic peroxides which have been incorporated in the resins. For example, in the irradiation method not only is a special costly apparatus required, but also it is very difficult to accomplish uniform crosslinking in the case of molded products of thick walls or of complicated shapes. Thus, this method is not practicable. On the other hand, the method using organic peroxides often results in poor quality of the molded or formed products and, more seriously, sometimes in failure of the molding process to continue because the crosslinking based on decomposition of the peroxides occurs during the molding step. Furthermore, on conducting the crosslinking step after the molding, a high temperature-decomposable peroxide which can withstand the molding temperature must be used, and thus the molded product must be heated to a temperature higher than the molding temperature to decompose such peroxides. As a result, the molded product undergoes changes due to softening and also results in poor quality of the molded product.
Furthermore, there have been problems encountered in using an ethylene-unsaturated silane compound copolymer or an unsaturated silane compound-graftpolymerized polyolefin. More specifically, a vinylalkoxysilane such as vinyltrimethoxysilane or vinyltriethoxysilane has been used as the unsaturated silane compound in these cases. However, the composition wherein the vinylalkoxysilane is used is not always satisfactory in its crosslinking velocity and thus requires a long time for crosslinking. An improvement in this respect has been greatly desired.